git: add config file telling git-publish how to send patches

The "git-publish" tool is a useful git extension for sending patch
series for code review. It automatically creates versioned tags
each time code on a branch is sent, so that there is a record of
each version. It also remembers the cover letter so it does not
need re-entering each time the series is reposted.
With this config file present it is now sufficient[1] to run
$ git publish
to send all patches in a branch to the list for review, with the
correct subject prefix added for this non-core libvirt module.
[1] Assuming your $HOME/.gitconfig has an SMTP server listed
at least e.g.
[sendemail]
smtpserver = smtp.example.com
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>